High strength materials have been used for critical components in order to ensure the reliability of aircraft gas turbine engine. But it usually costs high to maintain their premium quality. Therefore, the optimum quality level of materials should be defined under operating condition by material property estimation technique in order to meet the requirement of reliability and economical efficiency. Most cast rotating parts of gas turbine engine have casting defects caused during the casting process. The casting defects less than certain acceptable limit can be permissible for usage. So, it is very important to understand material defect shape and distribution, and its effect on mechanical properties exactly. In this study, 17-4PH stainless steel specimens with variable internal casting defects were prepared to investigate the fatigue property characteristics. The defect fraction and condition were estimated in microstructure by statistical analysis. The correlation between estimated defect condition and fatigue property was discussed.